Due to the rise of melanoma and other skin related diseases, protection of the skin from the deleterious effects of the sun has become a priority in recent years. The quest for complete protection form the sun has lead to numerous research efforts of not only effective sun blockers but also products that are extremely efficient. A product that protects a wide variety of ultra violet (UV) radiation and is effective has become a priority. The effectiveness of sunscreen products are typically listed in terms of sun protection factor (SPF). The SPF can be altered by a variety of factors including the specific sunscreen agents chosen and the type of delivery systems (formula) chosen.
Sunscreen products work based on the ability of the sunscreen actives to absorb photons in the Ultraviolet B and Ultraviolet A range (UVB and UVA respectively). Simply put according to Beers Law Absorbance of light passing through a liquid is directly related to the concentration of absorbing material in the liquid. If one notes the published absorbance of sunscreen actives approved for use in the United States it can readily be deduced that many sunscreens utilized actives at levels many times greater than that should be necessary to obtain the desired SPF. This can be attributed to several factors but certainly one that is extremely important is the sunscreen active solvent system used in the formulations.
Sunscreens have been assigned Sun Protection Factor (SPF) values by the U.S. Food and Drug Administration (FDA) since 1978. SPF is a number that refers to the sunscreen product's ability to block UVB radiation. This number does not show the blockade against ultraviolet UVA radiation. Sunscreen products with SPFs of 2 to 50 are currently available. A sunscreen product with a SPF of 15 will protect your skin 15 times longer from UVB than if you did not have sunscreen applied. The exact amount of time will vary from person to person, the altitude, and proximity to the equator. SPF 15 will block 95% of the UVB wavelengths. SPF 30 does not work twice as well however, it will provide another 3% of protection.
Broad-spectrum sunscreens were developed to absorb both UVA and UVB energy. To achieve coverage over the UVA and UVB spectra, multiple sunscreens are selected both on the basis of absorbed wavelength range as well as other properties (i.e., water resistance, hypoallergenicity). A prevailing paradigm in sunscreen formulation has been “more is better”. Many follow the approach that high SPF or more Boots stars can best be achieved by including many sunscreens in high concentrations. Because many sunscreens have decreased performance characteristics (e.g., lower SPF) when exposed to natural light, adherents of this school of formulation add more sunscreen actives than should theoretically be required to achieve a certain SPF. In so doing, they compensate for the degradation that takes place in the laboratory setting. However, this reasoning is flawed. There is markedly more photodegradation in natural sunlight, causing the actual SPF realized by the consumer to be lower.
The “more is better” paradigm also overlooks the fact that among the degradation products in photolabile sunscreens are free radicals, which can cause damage to DNA and other cellular molecules. Over time, free radical damage may become irreversible and lead to disease including cancer. Moreover, to the extent that a sunscreen is photolabile under artificial light (e.g., JCIA, COLIPA), that same composition could undergo more photodegradation, and produce more free radicals, when exposed to UVR as well as infrared and visible light under ambient conditions. Thus, a third objective of the present invention is to identify a combination sunscreen composition where after irradiation under ambient light each sunscreen active is photostable and thereby minimize the formation of potentially harmful free radicals.
Until recently, there have been three major types of sunscreen formulations: water in oil emulsion, the oil in water emulsion and the alcohol based formulation. All have their respective advantageous and disadvantageous. There have been formulation problems in making high SPF systems using oil-based sprays, including: application onto the skin and skin feel.
It has recently been discovered that the solvent effects can have a dramatic effect upon the SPF of a given formulation of sunscreen product. Solvent effects can be used to improve has a formulation's efficiency. By efficiency is meant the SPF that is obtained with a given level of sunscreen agent. The key to increasing the SPF of a formulation without altering the type of concentration of sunscreen is selecting the proper solvent.
A very useful and efficient tool for improving the SPF and absorption of a particular sunscreen is to modify the environment (i.e. the solvent) in which the active ingredients are placed. The modification of the environment can have a drastic effect on the over all performance of the sunscreen ranging from SPF, excitation wavelength, water tolerance and flammability of sunscreen.
Typically organic sunscreens absorb UV radiation by promoting electrons into an excited state. The effectiveness of the organic sunscreens are based on a couple of factors: the amount of energy (wavelength) that is required to promote an electron and how long the electron stays promoted before returning to the ground state. There are several ways that electrons can return to the ground state; they release the “stored” energy all at once and emit a different frequency of energy (fluoresces), they can transfer their energy to another molecule, or they can dissipate the energy as thermal heat. All three of these ways are directly related to the solvent the active ingredients are contained in.
Avobenzone, a common organic sunscreen, undergoes a keto/enol tautomerization. When avobenzone is placed into a polar environment, i.e. alcohol, the equilibrium lies heavily on the enol side. This results in a boost in SPF of the avobenzone. Solvent molecules can also transfer hydrogen atoms with excited molecules and “trap” them in a non-excitable state. This leaves the active sunscreen unable to promote an electron and therefore unable to absorb UV radiation. Beyer et al. used Raman spectroscopy to show that the excited state of N,N-dimethyl-p-amino-benzoic acid can accept a proton from a polar solvent resulting in loss of conjugation throughout the molecule. The interaction between active ingredients and the solvent can be easily modified and adjusted to fit the products needs.
An effective way of affecting the performance of organic sunscreens is to load them into delivery systems. Delivery systems most commonly involve amphiphilic systems, emulsions or amphiphilic macromolecules. Both emulsions and macromolecules act in the same manor. In oil in water emulsion, there are pockets of hydrophobic oil contained in the core of micelles. When hydrophobic organic sunscreens are added into the emulsion, they migrate into the hydrophobic micelle cores and remain suspended in a unified matrix. This organization provides two major advantages, firstly they pockets of actives provide a big boost in SPF and lastly the organization prevents aggregation of the organics, this removes color from the sunscreens on the skin. Macromolecules and polymers respond in the same way as emulsions, with the major difference being that the hydrophobic and hydrophilic portions of the formula are covalently attached to each other.
There has been a long felt need to provide formulations in which the SPF is increased providing longer protection with the same concentration of sunscreen. The new paradigm is less is more, that is less sunscreen actives formulated with the proper additives will provide significantly increases protection. The ability to so formulate products has been heretofore unattainable before the compounds of the present invention were developed.